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Facebook is cutting police departments off from a vast trove of data that has been increasingly used to monitor protesters and activists.

The move, which the social network announced today, comes in the wake of concerns over law enforcement's tracking of protesters' social media accounts in places such as Ferguson, Missouri, and Baltimore.

It also comes at a time when chief executive Mark Zuckerberg says he is expanding the company's mission from merely "connecting the world" into friend networks to promoting safety and community.

Although the social network's core business is advertising, Facebook, along with Twitter and Facebook-owned Instagram, also provides developers access to users' public feeds. The developers use the data to monitor trends and public events.

For example, advertisers have tracked how and which consumers are discussing their products, while the Red Cross has used social data to get real-time information during disasters such as Hurricane Sandy.

But the social networks have come under fire for working with third parties who market the data to law enforcement. Last year, Facebook, Instagram and Twitter cut off access to Geofeedia, a start-up that shared data with law enforcement, in response to an investigation by the American Civil Liberties Union.

The ACLU published documents that made references to tracking activists at protests in Baltimore in 2015 after the death of a black man, Freddie Gray, while in police custody and also to protests in Ferguson, Missouri, in 2014 after the police shooting of Michael Brown, an unarmed black 18-year-old.

Today, Facebook updated its instructions for developers to say that they cannot "use data obtained from us to provide tools that are used for surveillance."

The company also said, in an accompanying blog post, that it had kicked other developers off the platform since it had cut ties with Geofeedia.

Mark Zuckerberg says he is expanding the company's mission from merely "connecting the world" into friend networks to promoting safety and community. Photo / AP

Until now, Facebook hasn't been explicit about who can use information that users post publicly. This can include a person's friend list, location, birthday, profile picture, education history, relationship status and political affiliation - if they make their profile or certain posts public.

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Some departments have praised the tools, which they say helps them fight crime - for example, if gang leaders publicly post references to their crimes.

In a statement about the changes, which were the results of several months of conversations with activists, the ACLU and other groups lauded Facebook's move as a "first step."

"We depend on social networks to connect and communicate about the most important issues in our lives and the core political and social issues in our country," Nicole Ozer, technology and civil liberties director at the ACLU of California, said in the statement.

"Now more than ever, we expect companies to slam shut any surveillance side doors and make sure nobody can use their platforms to target people of color and activists."

Some said Facebook hadn't gone far enough. "When technology companies allow their platforms and devices to be used to conduct mass surveillance of activists and other targeted communities, it chills democratic dissent and gives authoritarianism a license to thrive," Malkia Cyril, executive director and founder of the Center for Media Justice, said in the statement. "It's clear there is more work to be done to protect communities of color from social media spying, censorship and harassment."

The new policy language does not kick law enforcement off the platform. For one, the company cooperates with law enforcement on a case-by-case basis for help in solving crimes.

Police and federal agencies may still siphon people's feeds in cases of national disasters and emergencies, Facebook officials said. It was unclear how Facebook would decide which emergencies and public events would warrant monitoring citizens' data and which would constitute unreasonable "surveillance."

"Surveillance" was also not defined in the blog post, a potential gray area that outsiders can exploit. Facebook said it would continue to audit third parties for policy violations and require that developers disclose what they plan to do with data they are requesting access to.

Local police departments across the United States have spent roughly US$5 million (NZ$7.2m) on social media monitoring over the past several years, according to the Brennan Center for Justice. The relatively small amount shows how it is inexpensive to track and monitor the behavior of large numbers of people.